Field of the Invention
The present invention relates to an image forming apparatus using an electro-photographic system or an electrostatic recording system.
Description of the Related Art
In an image forming apparatus that uses an electro-photographic system or an electrostatic recording system, such as a copier and printer, an intermediate transfer system, which transfers a toner image formed on a photoreceptor to an intermediate transfer belt (primary transfer) and then transfers the toner image from the intermediate transfer belt to a transfer material (recording material) (secondary transfer) so as to output the image, is known. For the intermediate transfer belt, an endless belt type intermediate transfer belt is widely used. The methods used for cleaning the residual toner on the intermediate transfer belt are roughly classified into: a blade cleaning system; an electrostatic cleaning system; and a hybrid system which uses both of these systems.
The blade cleaning system (Japanese Patent Application Laid-Open No. 2009-288481) is a method of physically scraping off the residual toner on the intermediate transfer belt using a cleaning belt that is contacting the intermediate transfer belt. In the case of this cleaning system, a good cleaning performance can be expected at low cost, but a disadvantage is that the blade wears out due to long time use and is easily influenced by unevenness on the surface of the intermediate transfer belt, which makes it difficult to maintain a good cleaning performance for a long period of time.
In the case of the electrostatic cleaning system (Japanese Patent Application Laid-Open No. 2009-205012), the residual toner is charged to the reversed polarity state of the charged state at development using the charging unit to which voltage is applied. Then the residual toner charged to the reversed polarity is transferred back from the intermediate transfer belt to the photoreceptor in the next primary transfer step, and is collected by the cleaning unit which cleans the photoreceptor. This system is therefore called a “simultaneous transfer-and-cleaning system”.
An advantage of the electrostatic cleaning system is that cleaning is not affected very much by unevenness on the surface of the intermediate transfer belt, but if images are continuously formed, toner is deposited on the charging unit, and this toner must be cleared to maintain cleaning performance. To clean the charging unit in the electrostatic cleaning system, the attached toner is discharged from the charging unit by applying voltage having the same polarity as the toner, and collecting the discharged toner to the photoreceptor. Since the charging polarity of the toner immediately after the discharge is reversed polarity of the primary transfer voltage, the discharged toner cannot be collected to the photoreceptor by the primary transfer unit immediately after the discharge. Hence the discharged toner must be charged to the same polarity as the primary toner voltage by the charging unit again by the step of further rotating the intermediate transfer belt. This means that time to rotate the intermediate transfer belt is required only for this discharging step.
In the case of the hybrid type cleaning method (Japanese Patent Application Laid-Open No. 2000-131920), most of the residual toner on the intermediate transfer belt is removed by a cleaning blade disposed downstream of the secondary transfer unit in the rotation direction of the intermediate transfer belt. Residual toner that passed by the cleaning blade is charged by a charging unit, such as a conductive brush, disposed downstream of the cleaning blade in the rotation direction of the intermediate transfer belt, whereby simultaneous transfer-and-cleaning to the photoreceptor is performed. In this hybrid system, cleaning can be assisted by the charging unit, as mentioned above, even if the blade wears out by long time usage. Further, for the cleaning of the charging member, attached toner is discharged in the same manner as the electrostatic cleaning system and the discharged toner can be collected by the cleaning blade, hence the rotating time of the intermediate transfer belt decreases. As a consequence, the cleaning method of the hybrid system can implement: a shorter processing time (downtime) than the electrostatic cleaning method; and a better cleaning performance for a long period of time than the blade cleaning system.